Human teeth often need repair due to destructive forces of injury, caries and aging. The restoration of teeth frequently requires the replacement of a core filling material.
Silicate cements have been used in the past for the repair of teeth and have the good properties of low thermal expansion, high abrasion resistance when not attacked by acids, and the ability to afford some caries protection by the liberation of fluoride ions.
Polycarboxylate cements are noted for their hydrophilic properties, good adhesion to tooth structure and apparent blandness. Polycarboxylate cements are based on zinc oxide or magnesium oxide or tin oxide and an aqueous solution of polyacrylic acid or an acrylic acid copolymer with other unsaturated carboxylic acids.
One of the most widely used filling materials is composite resins but these frequently possess coefficients of thermal expansion which are two to three times that of tooth material. This is a significant disadvantage and may result in increased microleakage and may lead to recurrent caries.
Glass ionomer cement filling materials have been previously developed which have addressed some of the above disadvantages of composite resin. Glass ionomer cement has strength characteristics similar to those cited above for silicate cements but is more resistant to acid attack. It is also bland, like the polycarboxylate cements, but with the added advantage of translucency.
The setting or hardening reaction of glass ionomer compositions occurs when a water soluble polymer having pendent carboxylic acid groups reacts with an ion-leaching glass powder. In the setting reaction, the glass powder behaves like a base and reacts with the acidic polyelectrolyte, i.e., ionomer, to form a metal polysalt (ionic cluster) which acts as the binding matrix. Water serves as a reaction medium, facilitating ion transport in what is essentially an ionic reaction. The setting reaction is characterized to be a chemically cured system that proceeds automatically upon mixing the ionomer and glass powder in the presence of water. The mixtures set or react to form a gel-like material within a few minutes and this material further hardens rapidly to develop the desired strength. Tartaric acid and other chelating agents are useful for modifying the rate of setting to thereby provide expanded working times for the composites or cements.
The ability of glass ionomer cements or composites to leach fluoride ions and to bond to tooth structure are their main advantages, since these materials are dynamic in nature, capable of ion-exchange at the tooth surface. Their anti-cariogenic properties, combined with molecular attachment to structure, make them the material of choice for treating early carious lesions or patients with a high caries incidence. However, glass ionomer materials are inherently brittle and can be prone to porosity, a further cause of weakness. As a result, the use of traditional glass ionomers has been limited to anteriors, non-stress bearing areas in gingival erosion, abrasive cavities and fissures. Glass ionomer cement materials continue to have significant limitations for use in permanent posterior, particularly with regard to large restorations.
A major problem with commercially used polymers for glass ionomers, such as poly(acrylic acid) (I) or poly(acrylic acid-co-itaconic acid) (II), resides in the direct or very close attachment of all the acid (CO.sub.2 H) groups to the polymer backbone as shown below. ##STR1##
U.S. Pat. No. 4,663,409 teaches the use of amino acid based monomers for improving the properties of contact lenses.
Therefore, it would be desirable to produce glass ionomer cement materials with significantly enhanced physical properties, with retention of all the positive features of these dental materials.